1. Field of the Invention
The present invention relates to a pixel circuit and a driving method used in an Active Matrix Organic Light Emitting Display (referred to as “AMOLED”, hereinafter) and the like. An organic light emitting diode is also referred to as an organic EL element (referred hereinafter as “OLED”).
2. Description of the Related Art
There is no standard pixel circuit of AMOLED, so that each of the companies manufacturing AMOLED uses their original pixel circuits. A typical pixel circuit is provided with OLED, a drive transistor for driving the OLED, a plurality of transistors for switches, a capacitor, and the like.
In order to compensate variations and fluctuations of a threshold voltage of a driving transistor which supplies an electric current to the OLED in the pixel circuit, some techniques for detecting the threshold voltage is known (see Japanese Unexamined Patent Publication 2014-029533 (Patent Document 1) and Japanese Unexamined Patent Publication 2013-210407 (Patent Document 2), for example). The mainstreams of the techniques for detecting the threshold voltage are the following two types of techniques.
(1) A method with which: a gate terminal and a drain terminal are connected; the potential of a source terminal, for example, is fixed; and the potential of the gate terminal is changed by an electric current between the source and the drain to bring the voltage between the gate and the source automatically to be close to the threshold voltage (diode connection type). (2) A method with which: the potential of a gate terminal is fixed; the potential of a source terminal is changed by an electric current between the drain and the source to bring the voltage between the gate and the source automatically to be close to the threshold voltage (source follower type). The source follower type is advantageous in being able to detect the threshold voltage of a depression type transistor in which an electric current is flown even when the voltage between the gate and the source is 0 V.
However, the existing pixel circuit with a threshold voltage detecting function has following issues.
(1) When white display is shown after showing black display for a while, due to the hysteresis characteristic of the driving transistor, the screen does not change to white immediately but the time for several frames are required to turn to all-white display. This is called “image retention” in general (see Japanese Unexamined Patent Publication 2012-128386 (Patent Document 3), for example). In other words, the hysteresis characteristic of the driving transistor is initialized when an electric current is not flown to the driving transistor for a long time, so the threshold voltage shifts to a direction of increasing the electric current. Under such state, even if the gate-source voltage for white-display compensating the threshold voltage applies to the driving transistor, the electric current is decreased instantly due to the hysteresis characteristic. Therefore, the brightness of the original white display cannot be acquired.
(2) Due to leaked light emission in a non-emission period, the contrast deterioration occurs. The reason is that the electric current as following cases is flown into the OLED during the non-emission period so that invalid leaked light emission is generated. (a) The electric current for the driving transistor flows via the OLED in a threshold voltage detecting period. (b) The charged electric current of the capacitor flows via the OLED in a capacitor-reset period.
Next, the related techniques will be described. Reference numbers in FIG. 24A to FIG. 27B are directly employed from the publications for just explanation purpose, so that those reference numbers are irrelevant to the reference numbers of other drawings in this invention.
(Related Art 1)
Related Art 1 shown in FIG. 24A and FIG. 24B is depicted in FIG. 1 and FIG. 2 of Patent Document 1.
A pixel circuit 200 of Related Art 1 includes an OLED 10, a driving transistor 14, switching transistors 16, 18, a capacitor 12, and the like, and discloses a following subject and feature. The pixel circuit 200 is of a source follower type, in which the switching transistor 18 is connected to the anode of the OLED 10. The pixel circuit 200 does not detect a threshold voltage at which an electric current does not flow. The pixel circuit 200 flows a prescribed bias current to the driving transistor 14 via a bias line IBIAS to adjust the potential of a source terminal B11. The potential of the source terminal B11 is applied to the OLED 10 when the supply voltage VDD is not decreased at programming cycles X11 and X12. Therefore, leaked light emission is generated, and the electric current flown to the driving transistor 14 cannot be brought up to the prescribed bias current.
(Related Art 2)
Related Art 2 shown in FIG. 25A and FIG. 25B is depicted in FIG. 26 and FIG. 27 of Patent Document 1.
A pixel circuit 420 of Related Art 2 includes an OLED 422, a driving transistor 426, switching transistors 428, 430, 432, 434, 436, a capacitor 424, and the like, and discloses a following subject and feature. The pixel circuit 420 is of a source follower type, in which the switching transistor 436 is connected to the source terminal of the driving transistor 426. The switching transistor is not connected to the anode of the OLED 422. The pixel circuit 420 does not detect a threshold voltage. The pixel circuit 420 flows prescribed bias current to the driving transistor 426 via a bias line Ibias to adjust the potential of a source terminal. The prescribed bias current will flow to the OLED 422 in a non-emission period X71, and leaked light emission is generated.
(Related Art 3)
Related Art 3 shown in FIG. 26A and FIG. 26B is depicted in FIG. 16 and FIG. 25 of Patent Document 1.
A pixel circuit 210 of Related Art 3 includes an OLED 90, a driving transistor 96, switching transistors 98, 100, 102, 104, capacitors 92, 94, and the like, and discloses a following subject and feature. The pixel circuit 210 is of a diode connection type, in which the switching transistor 96 is connected to the anode of the OLED 90. The pixel circuit 210 does not detect a threshold voltage. In the pixel circuit 210, a prescribed bias current flows to the driving transistor 96 via a bias line IBIAS to adjust the voltage between the gate and the drain. The voltage of the node C32 will apply to the OLED 90 if the supply voltage VDD is not decreased at a programming cycle X61. Therefore, leaked light emission is generated, and the prescribed bias current cannot be flown to the driving transistor 96.
(Related Art 4)
Related Art 4 shown in FIG. 27A and FIG. 27B is depicted in FIG. 2 and FIG. 4 of Patent Document 2.
A pixel circuit 2A of Related Art 4 includes an OLED 3, a driving transistor T2, switching transistors T1, T3, T4, T5, T6, a capacitor C1, and the like, and discloses a following subject and feature. The pixel circuit 2A is of a diode connection type, in which the switching transistor T6 is connected to the anode terminal of the OLED 3. The switching transistor T6 is used only for fixing the potential of the anode terminal but not used for resetting the terminal of the driving transistor T2 and for preventing image retention. That is, there is no simultaneous conduction of the switching transistor T6 and the switching transistor T4.
Therefore the present invention provides a pixel circuit and the like preventing the image retention as firstly and the contrast deterioration caused due to leaked light emission in a non-emission period as secondly.